In the use of a synchronous motor, it is required to boost the total efficiency of a synchronous motor drive system for driving the synchronous motor. To achieve a high efficiency, technology called Pulse Width Modulation control (hereinafter referred to as “the PWM control”) is often used in three-phase inverters for supplying driving power to a synchronous motor, for example.
Meanwhile, due to development of semiconductor technology, synchronous motors have been commonly used as vehicle motors in recent years, in view of their reliability, controllability, efficiency, etc., even though the power source of a vehicle motor is a battery, which is a DC power source. In such cases, a high torque is required as well as a high efficiency.
Generally, to increase the torque of a synchronous motor, it is necessary to increase the frequency of the motor current. In the case of a three-phase inverter performing the PWM control, however, the frequency of switching operations also increases as the current frequency increases. This increases the switching loss, which is problematic.
To decrease the switching loss, Patent Literature 1 and Patent Literature 2 disclose technology for driving a three-phase inverter with a low carrier frequency while the rotation speed is low, and driving the three-phase inverter with a high carrier frequency while the rotation speed is high, for example. Such technology reduces the switching loss, which is problematic in increasing the frequency of an inverter, according to the drive state of a motor.